The present invention relates to optical fiber transmission and more specifically to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
The index profile of optical fibers is generally described by the shape of the graph of the function that associates the refractive index of the fiber with its radius. It is conventional to plot the distance r from the center of the fiber on the abscissa axis and the difference between the refractive index and the refractive index of the cladding of the fiber on the ordinate axis. The expressions xe2x80x9cstepxe2x80x9d index profile, xe2x80x9ctrapeziumxe2x80x9d index profile, and xe2x80x9ctrianglexe2x80x9d index profile are therefore used with reference to graphs that are respectively step-shaped, trapezium-shaped and triangular. These curves are generally representative of the theoretical or ideal profile of the fiber and fiber fabrication constraints can yield a perceptibly different profile.
It is advantageous to manage chromatic dispersion in new high bit rate wavelength division multiplexed (WDM) transmission networks, especially for bit rates greater than or equal to 40 Gbit/s or 160 Gbit/s; the objective, in order to limit pulse widening, is to obtain substantially zero cumulative chromatic dispersion over the link, for all wavelengths of the multiplex. A cumulative dispersion value of a few tens of ps/nm is generally acceptable. In the vicinity of wavelengths used in the system, it is also beneficial to avoid zero values of the local chromatic dispersion, for which the non-linear effects are strongest. Furthermore, it is also beneficial to limit the cumulative chromatic dispersion slope over the range of the multiplex to prevent or limit distortion between multiplex channels. The chromatic dispersion slope is conventionally the derivative of chromatic dispersion with respect to wavelength.
Step index fibers, also known as single mode fibers (SMF), are conventionally used as line fibers in optical fiber transmission systems. The applicant""s ASMF 200 step index monomode fiber has a chromatic dispersion cancellation wavelength xcex0 from 1300 to 1320 nm and a chromatic dispersion less than or equal to 3.5 ps/(nm.km) in a range from 1285 to 1330 nm and of the order of 17 ps/(nm.km) at 1550 nm. The chromatic dispersion slope at 1550 nm is of the order of 0.06 ps/(nm2.km).
Dispersion shifted fibers (DSF) have also become available. At the transmission wavelength at which they are used, which is generally different from the wavelength of 1.3 xcexcm for which the dispersion of silica is substantially zero, the chromatic dispersion is substantially zero; in other words, the non-zero chromatic dispersion of the silica is compensatedxe2x80x94whence the use of the term xe2x80x9cshiftedxe2x80x9dxe2x80x94by increasing the index difference An between the core of the fiber and the optical cladding. The index difference causes the wavelength at which zero chromatic dispersion is obtained to be offset; it is achieved by introducing dopants into the preform, during fabrication, for example by a modified chamical vapor deposition (MCVD) process known in the art, and which is not described in more detail here.
Non-zero dispersion shifted fibers (NZ-DSF+) are dispersion shifted fibers having positive non-zero chromatic dispersion at the wavelengths at which they are used, typically around 1550 nm. At these wavelengths these fibers have a low chromatic dispersion, typically less than 11 ps/(nm.km) and a chromatic dispersion slope from 0.04 to 0.1 ps/(nm2.km) at 1550 nm.
The document FR-A2 790 107 proposes a line fiber especially suitable for dense wavelength division multiplex (DWDM) transmission with a channel spacing of 100 GHz or less for a bit rate per channel of 10 Gbit/s; at a wavelength of 1550 nm, this fiber has an effective surface area greater than or equal to 60 xcexcm2, a chromatic dispersion from 6 to 10 ps/(nm.km), and a chromatic dispersion slope less than 0.07 ps/(nm2.km).
French patent application number 00/02316 filed Feb. 24, 2000 whose title in translation is xe2x80x9cAn optical fiber exhibiting monomode behavior in-cable for wavelength division multiplex optical fiber transmission networksxe2x80x9d, proposes a line fiber which has, at a wavelength of 1550 nm, a chromatic dispersion from 5 to 11 ps/(nm.km), a ratio of chromatic dispersion to chromatic dispersion slope from 250 to 370 nm, and a ratio of the square of the effective surface area to the chromatic dispersion slope greater than 8xc3x97104 xcexcm2.nm2 km/ps. That line fiber has a range of use from 1300 to 1625 nm. In one example described in the above application, its dispersion is compensated by dispersion compensating fiber having a chromatic dispersion of xe2x88x92100 ps/(nm.km) and a ratio of chromatic dispersion to chromatic dispersion slope of 260 nm.
Using short lengths of dispersion compensating fiber (DCF) to compensate chromatic dispersion and chromatic dispersion slope in SMF or NZ-DSF+ used as line fiber is known in the art. One example of a transmission system in which chromatic dispersion in a SMF line fiber is compensated using DCF is described in M. Nishimura et al., xe2x80x9cDispersion compensating fibers and their applicationsxe2x80x9d, OFC""96 Technical Digest ThAl. Such use of dispersion compensating fiber is also mentioned in L. Grxc3xcner-Nielsen et al., xe2x80x9cLarge volume Manufacturing of dispersion compensating fibersxe2x80x9d, OFC""98 Technical Digest TuD5. The above articles, and other prior art documents, propose choosing the dispersion compensating fiber as a function of the line fiber so that the ratios of chromatic dispersion to chromatic dispersion slope of the compensating fiber and the line fiber are substantially equal.
DCF are also described in various patents. In the vicinity of a wavelength of 1550 nm they have a negative chromatic dispersion to compensate the cumulative chromatic dispersion in the line fiber, and can also have a negative chromatic dispersion slope to compensate the positive chromatic dispersion slope of the line fiber. The documents U.S. Pat. No. 5,568,583 and U.S. Pat. No. 5,361,319 propose a DCF for compensating chromatic dispersion in a SMF which has a dispersion of the order of 17 ps/(nm.km).
The document WO-A-99 13366 proposes a dispersion compensating fiber that it is intended to be used in compensation modules to compensate the chromatic dispersion and the chromatic dispersion slope of a Lucent xe2x80x9cTrue Wavexe2x80x9d fiber; the fiber has a chromatic dispersion from 1.5 to 4 ps/(nm.km) and a chromatic dispersion slope of 0.07 ps/(nm2.km). One embodiment of the proposed dispersion compensating fiber has a chromatic dispersion of xe2x88x9227 ps/(nm.km) and a chromatic dispersion slope of xe2x88x921.25 ps/(nm2.km).
The above dispersion compensating fibers are suitable for transmission systems operating in the C band, i.e. from 1530 to 1565 nm, or around 1550 nm. They are not suitable for compensating chromatic dispersion and chromatic dispersion slope in transmission systems operating in the C and L bands at the same time. In this context, the term xe2x80x9cL bandxe2x80x9d refers to a range of wavelengths above the C band, up to wavelengths of the order of 1610 or 1620 nm. A transmission system in the C and L bands therefore typically uses wavelengths from 1530 to 1610 nm.
A French patent application filed Oct. 26, 2000 by the applicant, whose title in translation is xe2x80x9cAn optical fiber for in-line compensation of chromatic dispersion of a positive chromatic dispersion optical fiberxe2x80x9d, describes an optical fiber that is used to compensate chromatic dispersion and chromatic dispersion slope in C, S and/or L band transmission systems, but for chromatic dispersions from xe2x88x9212 ps/nm-km to xe2x88x924 ps/nm-km.
The invention proposes a new fiber that can in particular be used to compensate chromatic dispersion in a C and L band transmission system. It is suitable for all SMF and for all NZ-DSF line fibers. The invention further proposes a new criterion for optimizing dispersion compensating fibers relative to line fibers, to ensure good compensation of dispersion and dispersion slope in both the C and L bands.
To be more precise, the invention proposes an optical fiber transmission system having a line fiber section and a compensation fiber section for compensating chromatic dispersion in the line fiber section, in which system the chromatic dispersion C and the chromatic dispersion slope Cxe2x80x2 of the dispersion compensating fiber satisfy the following condition for a wavelength from the range of wavelengths used in the transmission system:                                                         C              g                                      C              g              xe2x80x2                                xc3x97                      1                          1              +                                                C                  cum                                /                                  (                                                                                    L                        g                                            ·                                              C                        g                        xe2x80x2                                            ·                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    λ                                    )                                                                    ≤                  C                      C            xe2x80x2                          ≤                                            C              g                                      C              g              xe2x80x2                                xc3x97                      1                          1              -                                                C                  cum                                /                                  (                                                                                    L                        g                                            ·                                              C                        g                        xe2x80x2                                            ·                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    λ                                    )                                                                                        (        1        )            
where Cg is the chromatic dispersion of the line fiber, Cxe2x80x2g is the derivative of the chromatic dispersion of the line fiber with respect to wavelength, Ccum is the upper limit of the modulus of the cumulative chromatic dispersion permitted in the line fiber section, and xcex94xcex is half the width of the range of wavelengths used in the transmission system, and in which system the chromatic dispersion slope Cxe2x80x2 and the first derivative Cxe2x80x3 of that slope with respect to wavelength of the dispersion compensating fiber satisfy the following condition for a wavelength substantially in the middle of the range of wavelengths used in the transmission system:                               "LeftBracketingBar"                                    C              xe2x80x2                                      C              xe2x80x2xe2x80x2                                "RightBracketingBar"                ≥                  Max          ⁡                      (                                                            Δ                  ⁢                                      xe2x80x83                                    ⁢                  λ                                2                            xc3x97                                                (                                                                                                              C                          ·                                                      C                            g                            xe2x80x2                                                                                                                                C                            xe2x80x2                                                    ·                                                      C                            g                                                                                              xc3x97                                              (                                                  1                          ±                                                                                    C                              cum                                                                                                                                                        L                                  g                                                                ·                                                                  C                                  g                                  xe2x80x2                                                                ·                                Δ                                                            ⁢                                                              xe2x80x83                                                            ⁢                              λ                                                                                                      )                                                              -                    1                                    )                                                  -                  1                                                      )                                              (        2        )            
The constraints (1) and (2) are advantageously satisfied for a wavelength substantially in the middle of the range of wavelengths used in the transmission system; in this case, the modulus of the difference between said substantially middle wavelength and the middle wavelength of the range of wavelengths used in the transmission system preferably differs by less than 10% from the range of wavelengths used in the transmission system.
The dispersion compensating fiber can have one or more of the following propagation characteristics:
a chromatic dispersion less than xe2x88x9240 ps/(nm.km), or even less than xe2x88x9245 ps/(nm.km), at 1550 nm;
bending losses less than or equal to 400 dB/m, or even less than or equal to 100 dB/m, when wound around a 10 mm radius former, and throughout the range of wavelengths used in the transmission system;
an effective surface area greater than or equal to 12 xcexcm2 at 1550 nm;
a polarization mode dispersion less than or equal to 0.5 ps/kmxc2xdat 1550 nm;
an attenuation less than 1 dB/km, or even 0.8 dB/km, at 1550 nm;
The ratio between the upper limit of the modulus of the cumulative chromatic dispersion Ccum and the length Lg of a line fiber section is advantageously less than 0.5 ps/(nm.km), or even 0.1 ps/(nm.km).
The line fiber can be a step index fiber or a dispersion shifted fiber with positive chromatic dispersion.
The dispersion compensating fiber preferably has a rectangle with buried trench and ring index profile or a trapezium with buried trench and ring index profile.
The invention also proposes an optical fiber having a chromatic dispersion C less than xe2x88x9240 ps/(nm.km), or even less than xe2x88x9245 ps/(nm.km), at 1550 nm, a chromatic dispersion slope Cxe2x80x2 and a first derivative Cxe2x80x3 of chromatic dispersion slope with respect to wavelength satisfying the conditions:             a      xc3x97              1                  1          +          b                      ≤          C              C        xe2x80x2              ≤          a      xc3x97              1                  1          -          b                      and            "LeftBracketingBar"                        C          xe2x80x2                          C          xe2x80x2xe2x80x2                    "RightBracketingBar"        ≥          Max      ⁡              (                  c          xc3x97                                    (                                                                    C                                          C                      xe2x80x2                                                        ⁢                                      1                    a                                    xc3x97                                      (                                          1                      ±                      b                                        )                                                  -                1                            )                                      -              1                                      )            
for a wavelength in the vicinity of 1570 nm, where a, b and c are parameters satisfying the conditions:
280xe2x89xa6axe2x89xa6360 nm,
0.2xe2x89xa6bxe2x89xa60.25, and
cxe2x89xa625 nm.
This fiber can have the propagation characteristics indicated above for the dispersion compensating fiber. It can have a rectangle with buried trench and ring index profile or a trapezium with buried trench and ring index profile.
The index profile can then have one or more of the following characteristics:
the difference (xcex94n2) between the index of the rectangle or the trapezium and index of the cladding is from 18xc3x9710xe2x88x923 to 30xc3x9710xe2x88x923 and the radius (r1) of the part of the fiber having an index higher than the cladding is from 1.2 to 2.1 xcexcm;
the difference (xcex94n2) between the index of the depleted trench and the index of the cladding is from xe2x88x928.5xc3x9710xe2x88x923 to xe2x88x924.5xc3x9710xe2x88x923 and the outside radius (r2) of said trench is from 3.4 to 6.1 xcexcm;
the difference (xcex94n3) between the index of the ring and the index of the cladding is from 1xc3x9710xe2x88x923 to 15xc3x9710xe2x88x923 and the outside radius (r3) of said ring is from 5.9 to 8 xcexcm;
twice the integral of the product of the radius and the index between a zero radius and the outside radius (r1) of the central part of the fiber having an index higher than the cladding is from 48xc3x9710xe2x88x923 to 65xc3x9710xe2x88x923 xcexcm2;
three times the integral of the product of the square of the radius and the index between a zero radius and the outside radius (r1) of the central part of the fiber having an index higher than the cladding is greater than 110xc3x9710xe2x88x923 xcexcm3;
twice the integral of the product of the radius and the index between the outside radius (r1) of the central part of the fiber having an index higher than the cladding and the inside radius (r2) of the ring is from xe2x88x92155xc3x9710xe2x88x923 to xe2x88x9275xc3x9710xe2x88x923 xcexcm2;
twice the integral of the product of the radius and the index between the inside radius (r2) and the outside radius (r3) of the ring is from 70xc3x9710xe2x88x923 to 210xc3x9710 xcexcm2;
twice the integral of the product of the radius and the index between a zero radius and the outside radius (r3) of the ring is greater than 20xc3x9710xe2x88x923 xcexcm2.
The invention further proposes an optical fiber having a chromatic dispersion C less than xe2x88x9240 ps/(nm.km), or even less than xe2x88x9245 ps/(nm.km), at 1550 nm, a chromatic dispersion slope Cxe2x80x2 and a first derivative Cxe2x80x3 of chromatic dispersion slope with respect to wavelength satisfying the conditions:             a      xc3x97              1                  1          +          b                      ≤          C              C        xe2x80x2              ≤          a      xc3x97              1                  1          -          b                      and            "LeftBracketingBar"                        C          xe2x80x2                          C          xe2x80x2xe2x80x2                    "RightBracketingBar"        ≥          Max      ⁡              (                  c          xc3x97                                    (                                                                    C                                          C                      xe2x80x2                                                        ⁢                                      1                    a                                    xc3x97                                      (                                          1                      ±                      b                                        )                                                  -                1                            )                                      -              1                                      )            
for a wavelength in the vicinity of 1570 nm, where a, b and c are parameters verifying the conditions:
120xe2x89xa6axe2x89xa6200 nm,
0.2xe2x89xa6bxe2x89xa60.25, and
cxe2x89xa625 nm.
This fiber can have the propagation characteristics indicated above for the dispersion compensating fiber. It can have a rectangle with buried trench and ring index profile or a trapezium with buried trench and ring index profile.
The index profile can then have one or more of the following characteristics:
the difference (xcex94n1) between the index of the rectangle or the trapezium and the index of the cladding is from 18xc3x9710xe2x88x923 to 30xc3x9710xe2x88x923 and the radius (r1) of the part of the fiber having an index higher than the cladding is from 1.2 to 1.9 xcexcm;
the difference (xcex94n2) between the index of the depleted trench and the index of the cladding is from xe2x88x9210xc3x9710xe2x88x923 to xe2x88x925.8xc3x9710xe2x88x923 and the outside radius (r2) of said trench is from 3.4 to 6.1 xcexcm;
the difference (xcex94n3) between the index of the ring and the index of the cladding is from 1xc3x9710xe2x88x923 to 17.5xc3x9710xe2x88x923 and the outside radius (r3) of said ring is from 6.25 to 8.3 xcexcm;
twice the integral of the product of the radius and the index between a zero radius and the outside radius (r1) of the central part of the fiber having an index higher than the cladding is from 50xc3x9710xe2x88x923 to 60xc3x9710xe2x88x923 xcexcm 2;
twice the integral of the product of the radius and the index between a zero radius and the outside radius (r1) of the central part of the fiber having an index higher than the cladding is from 50xc3x9710xe2x88x923 to 60xc3x9710xe2x88x923 xcexcm2;
three times the integral of the product of the square of the radius and the index between a zero radius and the outside radius (r1) of the central part of the fiber having an index higher than the cladding is less than 110xc3x9710xe2x88x923 xcexcm3;
twice the integral of the product of the radius and the index between the outside radius (r1) of the central part of the fiber having an index higher than the cladding and the inside radius (r2) of the ring is from xe2x88x92230xc3x9710 to xe2x88x92145xc3x9710xe2x88x923 xcexcm2;
twice the integral of the product of the radius and the index between the inside radius (r1) and the outside radius (r3) of the ring is from xe2x88x92180xc3x9710xe2x88x923 to xe2x88x92260xc3x9710 xcexcm2;
twice the integral of the product of the radius and the index between a zero radius and the outside radius (r3) of the ring is from 20xc3x9710xe2x88x923 xcexcm2 to 145xc3x9710xe2x88x923 xcexcm2.